Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbomachines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel injectors in the combustors where the compressed working fluid mixes with fuel before igniting to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Combustion instabilities may occur during operation when one or more acoustic modes of the gas turbine are excited by the combustion process. For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel/air ratio fluctuation in the flame. When the resulting fuel/air ratio fluctuation and the acoustic pressure pulsations have a certain phase behavior (e.g., in-phase or approximately in-phase), a self-excited feedback loop may result. This mechanism, and the resulting magnitude of the combustion dynamics, depends at least in part on the delay between the time that the fuel is injected through the fuel ports and the time when the fuel reaches the combustion chamber and ignites, defined as “convective time” (tau). Generally, there is an inverse relationship between convective time and frequency: that is, when the convective time increases, the frequency of the combustion instabilities decreases; and when the convective time decreases, the frequency of the combustion instabilities increases.
At particular operating conditions, combustion dynamics at specific frequencies and with sufficient amplitudes, which are in-phase and coherent, may produce undesirable sympathetic vibrations in the turbine and/or other downstream components. Typically, this problem is managed by combustor tuning that limits the amplitude of the combustion dynamics in a particular frequency band. However, conventional combustor tuning may unnecessarily limit the operating range of the combustor.
Altering the frequency relationship between two or more combustors may reduce the coherence of the combustion system as a whole, diminishing any combustor-to-combustor coupling. In the context of this invention, coherence refers to the strength of the linear relationship between two (or more) dynamic signals, which is strongly influenced by the degree of frequency overlap between them. As the combustion dynamics frequency in one or more combustors is driven away from that of the other combustors, modal coupling of combustion dynamics is reduced, which, in turn, reduces the ability of the combustor tone to cause a vibratory response in downstream components.
Therefore, a system and method that reduces the modal coupling of combustion dynamics by varying the convective time between combustors would be useful in enhancing the thermodynamic efficiency of the combustors, protecting against accelerated wear, promoting flame stability, and/or reducing undesirable emissions over a wide range of operating levels.